A question mark is used because she is asking a question, even though it isn't syntactically a question, the purpose of this sentence is to check understanding.
In the telephone call, she begins "You know, I was organizing an amazing dinner party last night?". Syntactically this is a statement, but the purpose of this sentence is to check that the other person knows about the dinner party. Although we only hear one side of the conversation, we can imagine that the other person answers this question with "Yes, I knew that".
When a sentence is being used to get information, it can be marked with a question mark, even if it is not syntactically a question.
Since "You know" marks a sentence that gives information that you think the other person already knows, it is often used to check that the person really does share that information. So it is often used in this kind of informal question. In formal English you would ask "Do you know..."